Pressure Relief Reaction Forces – The Importance of ... · • Caesar II v 5.3 was used to detail ... • Vessel overpressure below hydrotest pressure typical ... The Importance

Pressure Relief Reaction Forces – The Importance of

Evaluating Existing Installations

John Burgess | Smith & Burgess

Agenda

• Installation Details – Inlet Piping Design – Piping Support – Administrative Controls

• Atmospheric Discharge – Dispersion Consideration – Liquids – Review Surrounding

Areas 2

Installation Details Inlet Piping Design – General Good Practices • Limit the inlet line losses to 3%

• Use full bore PSVs sparingly and knowingly • Ensure relief valve accessibility for maintenance • Ensure valves used for PSV isolation are full port

• Consider gate valves instead of ball valves for PSV inlets/outlets

• Audit the CSO/LO procedures • Ensure the outlet piping is free draining • Ensure that the outlet piping is supported • Ensure that the valve is vertical • Ensure that the valve disposition is

• Pointed Up • At least 10’ away from anything

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Installation Details

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Inlet Piping Design – Pressure Losses

Use of Full Bore Relief Valves • Good Engineering Practice

• Inlet losses > 3% of set • Valve Stable

• Manufacturers concerns • Valves may fail at ~ 7% to 10% • Capacity may suffer

These valves are not typical in downstream applications


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Inlet Piping Design – Pressure Losses • The Piping Must Support the Reaction Forces

A chemical Plant was suffering from nozzle failures, so we reviewed 189 installations for structural adequacy.

The failure criteria used was 70% of the Yield Stress limit.


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Inlet Piping Design – Pressure Losses • Installations were modeled using both

steady state and dynamic installation estimates.

• Caesar II v 5.3 was used to detail model ~ 15% of the valves.

⅓ of the valves required additional support, these were all API-526 Valves set below 500 psig.


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Inlet Piping Design – Administrative Controls

• Undersized • In 15 years of doing this work I have never seen an undersized

relief device causing a loss of containment. • Vessel overpressure below hydrotest pressure typical • MIDAS DB Search returns 0 Cases (Around 2000/2001)

• Isolated

• Overpressure potentially limitless • No capacity when isolated • Personal experience / knowledge is ~10 Cases • Seems to be increasing in frequency


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A Case Study from the CSB. • The rupture and release injured

six employees. • Operators closed an isolation

valve between the heat exchanger shell (ammonia cooling side) and a relief Valve

• Maintenance workers replaced the rupture disk on that day; however, the closed isolation valve was not reopened.

• The pressure in the heat exchanger shell continued climbing until it violently ruptured

Atmospheric Discharge

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General Considerations • Dispersion

• Dispersion characteristics need to be considered • Most liquids should not be discharged to atmosphere

• Review Areas Surrounding Vents

• Thermal Radiation Potential • Noise • Pollution Requirements


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Dispersion Considerations - general • Nothing within 10’

• Electrical Area Classification Requirements

• Nothing within 120 diameters, for systems with:

• Discharge point / PSV same diameter • Pop-Action PSVs • Limited Toxic Effects • MW < 50

• Toxic Considerations • 30:1 or 50:1 dilution @ 120 diameters • H2S Concentrations above 0.5 mole fraction may

dilute to above the IDLH (100 ppm)


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Dispersion Considerations - Details • Dispersion modeling required if the API guidance is not sufficient

• High concentrations of toxics (over 50x the limit) • Heavy gases • Low discharge velocities.

• Next Two Slides show the effects of Exit Velocity • Blue is “Okay”, between 10% and 50% of the LFL • Green “Concerning” between 50% of the LFL and the LFL • Yellow above the LFL and below the UFL

• High velocity discharge toxics may reach grade • ~500:1 dilution • H2S Concentrations above 5% (molar) may exceed IDLH


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General Considerations • Review Areas Surrounding

Vents • Thermal Radiation Potential • Noise • Toxic Effects • Pollution Requirements

• Consider the following • Are vents in areas used often by

personnel? • Are vents located near a property line? • If there were an emergency, could the

vent block egress? • Is the other equipment in the vent system

adequate?

Documents

Pressure Relief Reaction Forces – The Importance of ... · • Caesar II v 5.3 was used to detail ... • Vessel overpressure below hydrotest pressure typical ... The Importance